Relating in situ hydraulic conductivity, particle size and relative density of superficial deposits in a heterogeneous catchment

SummaryEstimating the permeability of superficial deposits is fundamental to many aspects of catchment science, but can be problematic where insufficient in situ measurements are available from pumping tests in piezometers. Consequently, common practice is to estimate permeability from the material description or, where available, particle size distribution using a formula such as Hazen. In this study, we examine the relationships between particle size, relative density and hydraulic conductivity in superficial deposits in Morayshire, Northern Scotland: a heterogeneous environment typical of many catchments subject to previous glaciations. The superficial deposits comprise glaciofluvial sands and gravels, glacial tills and moraines, raised marine sediments, and blown sands. Thirty-eight sites were investigated: hydraulic conductivity measurements were made using repeated Guelph permeameter measurements, cone resistance was measured in situ with a Panda dynamic cone penetrometer; material descriptions were made in accordance with BS5930:1999; and disturbed samples were taken for particle size analysis. Overall hydraulic conductivity (K) varied from 0.001 m/d to >40 m/d; glacial till had the lowest K (median 0.027 m/d) and glacial moraine the highest K (median 30 m/d). However, within each geological unit there was great variability in measured hydraulic conductivity values. Multiple linear regression of the data indicated that log d10 and relative density (indicated by cone resistance or BS5930:1999 soil state description) were independent predictors of log K and together gave a relationship with an R2 of 0.80. Material description using the largest fraction (e.g. sand or gravel) had little predictive power. Therefore, in heterogeneous catchments, the permeability of superficial deposits is most strongly related to the finest fraction (d10) and relative density of the material. In situ Guelph permeameter measurements at outcrops with good geological characterisation provide an easy and reliable method of determining the permeability of particular units of superficial deposits.

► We examine the permeability of superficial deposits in a heterogeneous catchment.
► K ranges from 0.001 to >40 m/d, highest in glacial moraine, lowest in till.
► MLR showed that K was related to log d10 and relative density with R2 of 0.80.
► Material description of largest fraction had little predictive power of K.